I keep looking at this thread's title feeling something is amiss.
I just realised it is the very concept of 'backup.'
I hear this term in firearms, usually a smaller caliber in a
compact package: What the late Colonel Cooper called " sending a
destroyer to 'back up' a battleship." It seems reasonable that
you should first be reasonably sure you won't lose or destroy
your primary blade. If you can't do that, then buy TWO to lose
or destroy. About the only use I see afield for backup blades
are the packers of tactical swords. You know; spreading peanut
butter, cutting paracord, making fuz sticks, striking firesteels.